There is a need for technologies for fast wireless communication in particular at 60 GHz and above, involving high gain antennas, intended for consumer market, so low-cost manufacturability is a must. The consumer market prefers flat antennas, and these can only be realized as flat planar arrays, and the wide bandwidth of these systems require corporate distribution network. This is a completely branched network of lines and power dividers that feed each element of the array with the same phase and amplitude to achieve maximum gain.
A common type of flat antennas is based on a microstrip antenna technology realized on printed circuits boards (PCB). The PCB technology is well suited for mass production of such compact lightweight corporate-fed antenna arrays, in particular because the components of the corporate distribution network can be miniaturized to fit on one PCB layer together with the microstrip antenna elements. However, such microstrip networks suffer from large losses in both dielectric and conductive parts. The dielectric losses do not depend on the miniaturization, but the conductive losses are very high due to the miniaturization. Unfortunately, the microstrip lines can only be made wider by increasing substrate thickness, and then the microstrip network starts to radiate, and surface waves starts to propagate, both destroying performance severely.
There is one known PCB-based technology that have low conductive losses and no problems with surface waves and radiation. This is referred to by either of the two names substrate-integrated waveguide (SIW), or post-wall waveguide as in [1]. We will herein use the term SIW only. However, the SIW technology still has significant dielectric losses, and low loss dielectric materials are very expensive and soft, and therefore not suitable for low-cost mass production. Therefore, there is a need for better technologies.
Thus, there is a need for a flat antenna for high frequencies, such as at or above 60 GHz, and with reduced dielectric losses and problems with radiation and surface waves. In particular, there is a need for a PCB based technology for realizing corporate distribution networks at 60 GHz or above that do not suffer from dielectric losses and problems with radiation and surface waves.
The gap waveguide technology is based on Prof. Kildal's invention from 2008 & 2009 [2], also described in the introductory paper [3] and validated experimentally in [4]. This patent application as well as the paper [5] describes several types of gap waveguides that can replace microstrip technology, coplanar waveguides, and normal rectangular waveguides in high frequency circuits and antennas.
The gap waveguides are formed between parallel metal plates. The wave propagation is controlled by means of a texture in one or both of the plates. Waves between the parallel plates are prohibited from propagating in directions where the texture is periodic or quasi-periodic (being characterized by a stopband), and it is enhanced in directions where the texture is smooth like along grooves, ridges and metal strips. These grooves, ridges and metal strips form gap waveguides of three different types: groove, ridge and microstrip gap waveguides [6], as described also in the original patent application [2].
The texture can be a periodic or quasi-periodic collection of metal posts or pins on a flat metal surface, or of metal patches on a substrate with metallized via-holes connecting them to the ground plane, as proposed in [7] and also described in the original patent application [2]. The patches with via-holes are commonly referred to as mushrooms.
A suspended (also called inverted) microstrip gap waveguide was presented in [8] and is also inherent in the descriptions in [6] and [7]. This consists of a metal strip that is etched on and suspended by a PCB substrate resting on top of a surface with a regular texture of metal pins. This substrate has no ground plane. The propagating quasi-TEM wave-mode is formed between the metal strip and the upper smooth metal plate, thereby forming a suspended microstrip gap waveguide.
This waveguide can have low dielectric and conductive losses, but it is not compatible with PCB technology. The textured pin surface could be realized by mushrooms on a PCB, but this then becomes one of two PCB layers to realize the microstrip network, whereby it would be much more costly to produce than gap waveguides realized only using one PCB layer. Also, there are many problems with this technology: It is difficult to find a good wideband way of connecting transmission lines to it from underneath.
The microstrip gap waveguide with a stopband-texture made of mushrooms were in [9] realized on a single PCB. This PCB-type gap waveguide is called a microstrip ridge gap waveguide, because the metal strip must have via-holes in the same way as the mushrooms.
A quasi-planar inverted microstrip gap waveguide antenna is described in [10]-[12]. It is expensive both to manufacture the periodic pin array under the microstrip feed network on the substrate located directly upon the pin surface, and the radiating elements which in this case were compact horn antennas.
A small planar array of 4×4 slots were presented in [13]. The antenna was realized as two PCBs, an upper one with the radiating slots realized as an array of 2×2 subarrays, each consisting of 2×2 slots that are backed by an SIW cavity. Each of the 4 SIW cavities was excited by a coupling slot fed by a microstrip-ridge gap waveguide in the surface of a lower PCB located with an air gap below the upper radiating PCB. It was very expensive to realize the PCBs with sufficient tolerances, and in particular to keep the air gap with constant height. The microstrip-ridge gap waveguide also requires an enormous amount of thin metallized via holes that are very expensive to manufacture. In particular, the drilling is expensive.
There is therefore a need for a new waveguide and RF packaging technology that have good performance and in addition is cost-efficient to produce.